Polycrystalline diamond compact ("PDC") cutters have been utilized in earth-boring drill bits. Typically, PDC bits include an integral bit body which may be made of steel or fabricated of a hard matrix material such as tungsten carbide (WC). A plurality of diamond cutter devices, e.g., PDC cutters, are mounted along the exterior face of the bit body. Each diamond cutter has a stud portion which typically is brazed in a recess or pocket in the exterior face of the bit body.
The PDC cutters are positioned along the leading edges of the bit body so that, as the bit body is rotated in its intended direction of use, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure and loss of one or more of the PDC cutters.
While steel body bits have toughness and ductility properties which render them resistant to cracking and failure due to impact forces generated during drilling, steel is more susceptible to erosive wear caused by high-velocity drilling fluids and formation fluids which carry abrasive particles, such as sand, rock cuttings, etc. Generally, steel body PDC bits are coated with a more erosion-resistant material, such as tungsten carbide, to improve their erosion resistance. However, tungsten carbide and other erosion-resistant materials are relatively brittle. During use, a thin coating of the erosion-resistant material may crack, peel off or wear, revealing the softer steel body which is then rapidly eroded. This can lead to loss of PDC cutters as the area around the cutter is eroded away, causing the bit to fail.
Tungsten carbide or other hard metal matrix body bits have the advantage of higher wear and erosion resistance. The matrix bit generally is formed by packing a graphite mold with tungsten carbide powder and then infiltrating the powder with a molten copper-based alloy binder. For example, macro-crystalline tungsten carbide and cast tungsten carbide have been used to fabricate the bit body. Macro-crystalline tungsten carbide is essentially stoichiometric WC which is, for the most part, in the form of single crystals. Some large crystals of macro-crystalline WC are bi-crystals. Cast tungsten carbide, on the other hand, is a eutectic two-phase carbide composed of WC and W.sub.2 C. There can be a continuous range of compositions therebetween. Cast tungsten carbide typically is frozen from the molten state and comminuted to a desired particle size.
However, a bit body formed of the aforementioned tungsten carbide or other hard metal matrix materials may be brittle and may crack upon being subjected to impact and fatigue forces encountered during drilling. Additionally, the braze joints between the matrix material and the PDC cutters may crack due to these forces. The formation and propagation of cracks in the matrix body and at the braze joints may result in the loss of one or more PDC cutters. A lost cutter may abrade against the bit, causing further accelerated bit damage.
For the foregoing reasons, there is a need for a new matrix-body composition for drill bits which offers improved ability to retain cutters while maintaining other desired properties, such as toughness, impact strength and wear resistance.